The primary method of manufacturing phosphoric acid is by digestion of phosphate rock with acid. This is commonly referred to as the "wet" process for phosphoric acid production. Apatite (calcium phosphate) is the most commercially valuable phosphate mineral. Sulfuric acid is the most commonly used acid for digestion. A by-product of wet process phosphoric acid production is the generation of a significant amount of calcium and magnesium sulfates (commonly referred to as phosgyp), and to a lesser degree other impure precipitates. The insoluble solids are typically removed by filtration, usually on a horizontal pan filter. The solids are then discharged as a waste product and the filtered acid is concentrated by evaporators with a clarification process between each stage. The final acid concentration is determined by the end use and can be as high as 67 to 76% P.sub.2 O.sub.5, which is known as superphosphoric acid. Fertilizer production accounts for about 90% of phosphate rock utilization.
After the phosphate matrix has been mined, it is beneficiated by washing, screening and/or flotation processes to concentrate the phosphatic material. The concentrated phosphate rock consists of phosphates of calcium, magnesium and the like with some organic as well as inorganic impurities. This material is dissolved by hot (140.degree.-190.degree. C.) sulfuric acid to yield a phosphoric acid solution of about 28% P.sub.2 O.sub.5. The sulfuric acid reacts with the cations in the phosphatic ore matrix, producing insoluble precipitates. The bulk of the insoluble solids consists of sulfates of calcium and magnesium which are removed by filtration prior to subsequent concentration of the acid. The efficiency of the filtration stage directly impacts the cost effectiveness of phosphoric acid production. Increases of 5-10% in the filtration rate are considered to be economically attractive. Many plants operate with filters at capacity and this stage is the limiting factor in phosphoric acid production. Polymeric filter aids offer a means of increasing production without capital investment. While a number of flocculants have been reported in the literature to improve filtration and acid clarification, there are no references known to the inventors relating to the use of the instant acrylic acid/acrylamido methylpropyl sulfonic acid-type polymers to improve gypsum filtration.
U.S. Pat. No. 4,800,071 discloses the use of sulfonated acrylamide and sulfonated acrylamide/acrylate polymers to aid in the filtration of gypsum in the "wet" process for production of phosphoric acid.
Flocculants are often used to aid the clarification process. See, for example, U.S. Pat. No. 3,644,091, which discloses the use of water soluble sulfonated polystyrenes having molecular weights of from about 1 to about 40 million as phosphoric acid clarification aids.
U.S. Pat. No. 4,291,005 discloses the use of acrylamide/acrylate polymers for settling suspended solids in phosphoric acid product solutions. The improvement wherein said flocculating agent is a copolymer consisting essentially of a predominant portion of acrylic acid units.
CA86(18):123780 and CA99(12):903x relate to the effect of polyacrylamide on the filtration rate of phosphogypsum.
CA91(24):195236r discloses the effect of various polyacrylamides and polyethylene oxide polymers on the filterability of phosphogypsum.
By contrast, the present invention relates to the use of designated high molecular weight carboxylic acid-type polymers with a portion of sulfonic functionality as filtration aids to improve the dewatering of gypsum slurries in the manufacture of wet process phosphoric acid.